Volumes of CO2 is not a rate of flow. If you wanted to measure the rate at which co2 is flowing out of a carbonator, you'd best look up fluid dynamics and gas law. There's something about the force of the pressure, the rate of flow, the pressure created and the diameter of the opening. http://en.wikipedia.org/wiki/Volumetric_flow_rate
Something or other like that.... Also: http://www.homebrewtalk.com/f14/volumes-co2-105955/
Your best (easiest) measurements are static measurements: Volumes of co2 currently in a beer/soda, or the psi created in a container (keg/bottle) based on the liquid and the gas. Especially because if you're pressurizing a closed system (into a keg), then the rate of flow changes (I think), due to the pressure exerted on both sides of the hose/regulator.

Thanks for answering I have mis represented my question. I am not concerned with flow rate and more interesting in controlling the volumes of CO2 (which is a specific measurement like that of lbs or grams...) of the carbonated water that is produced by the carbonator

In that case, you'd have a regulator for your co2 tank (which is pretty much a requirement to even use a co2 tank, otherwise you'd just lose all your co2 in an instant), then you'd use a chart like this: http://www.homebrew.com/pdfs/CO2chart.pdf locate the temperature of the liquid, and locate the requested volumes of co2, then you find what the psi would be, and you set your regulator to that desired amount.

Usually, for carbonated water/soda, you'd set your regulator to 30psi, then your water&gas mixture will result in 30 psi, which is... I can't recall the formula, but somewhere around 5 volumes of co2? (I just googled a volumes of co2 calculator for http://www.brewheads.com/forcecarb.php, and their calculation, in reverse... if I wanted 4.7 volumes of co2 in my liquid, then at a temperature of 32 degrees, I'd set my regulator to 30 psi. Or if I wanted 3 volumes of co2 in my liquid, I'd set my regulator at 14 psi)

I can't believe I didn't think of that. Henry's law would tell me the max amount of CO2 at a given pressure and temperature. So given the standard 100 PSI and a room temp of 70¤F this would achieve a co2 vol of about 7. This can't be right.

Flow through carbonators (Mccann) are a different animal. What's happening there is that liquid is flowing through a place that's 100psi, then sent out, then chilled. The end result is around the 30psi, since you've never let the water saturate to the 100psi point. I always give this reference http://truetex.com/carbonation.htm

However, in a non-flowing volume (keg/soda bottle) you're trapping everything to say 30 psi and shaking for a while then reaching the saturation point. You can set a higher (40-50) psi and shake less to reach 30psi, but it might accidentally go higher ending at... say 35-45. An even higher psi (100perhaps) and shake even less, but you may accidentally go over your desired 30 psi amount. The McCann units are designed to hit that certain point.